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Asteroid Belt
General Information The asteroid belt is the region of the Sol System located roughly between the orbits of the planets Mars and Jupiter . It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt is also termed the main asteroid belt or main belt to distinguish its members from other asteroids in the Solar System such as near-Earth asteroids and trojan asteroids . About half the mass of the belt is contained in the four largest asteroids, Ceres , Vesta , Pallas , and Hygiea . These have mean diameters of more than 400 km, while Ceres, the asteroid belt's only dwarf planet, is about 950 km in diameter. The remaining bodies range down to the size of a dust particle. The asteroid material is thinly distributed. Nonetheless, collisions between large asteroids do occur, and these can form an asteroid family whose members have similar orbital characteristics and compositions. It was once thought that collisions of asteroids produce a fine dust that forms a major component of the zodiacal light. However, Nesvorny and Jenniskens (2010 Astrophysical Journal) attributed 85 percent of the Zodiacal Light dust to fragmentations of Jupiter-family comets, rather than from comets and collisions between asteroids in the asteroid belt. Individual asteroids within the asteroid belt are categorized by their spectra, with most falling into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type). The asteroid belt formed from the primordial solar nebula as a group of planetesimals, the smaller precursors of the planets, which in turn formed protoplanets. Between Mars and Jupiter, however, gravitational perturbations from the giant planet imbued the protoplanets with too much orbital energy for them to accrete into a planet. Collisions became too violent, and instead of fusing together, the planetesimals and most of the protoplanets shattered. As a result, most of the asteroid belt's mass has been lost since the formation of the Solar System. Some fragments can eventually find their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are swept into other orbits. Classes of small Solar System bodies in other regions include the centaurs , Kuiper belt and scattered disk objects , and Oort cloud comets. The Asteroid Belt (Main Belt) has been mined and occupied from early on (2190 onward) and many thousand Asteroids are occupied or mined. The occupied Asteroids are represented via MABA What are Asterooids Asteroids are primoridal objects left over from the formation of the Solar System. While some have suggested that they are the remains of a protoplanet that was destroyed in a massive collision long ago, the prevailing view is that asteroids are leftover rocky matter that never successfully coalesced into a planet. Most planetary astronomers still believe that the planets of the Solar System formed from a nebula of gas and dust and ices that coalesced into a dusty disk around the developing Sun. Within the disk, tiny dust grains (and ices in the colder environs beginning around two AUs inside of Jupiter's orbit) coagulated into larger and larger bodies called planetesimals, many of which eventually accreted into planets over a period as long as a 100 million years. However, beyond the orbit of Mars , gravitational interference from Jupiter's huge mass prevented protoplanetary bodies from growing larger than about 1,000 km (620 miles), by sweeping many into pulverizing collisions as well as out into the Oort Cloud or beyond Sol's gravitational reach altogether. Main Asteroid Belt Most asteroids are rocky bodies that orbit the Sun between Mars and Jupiter in a "Main Asteroid Belt" that is centered around 2.7 times the Earth-Sun distance (astronomical unit or AU) from Sol. Two "clouds" of icy asteroids 60° ahead and behind Jupiter (and at or near Jupiter's orbital distance from the sun) are called "Jupiter Trojans" (diagram), while two similar objects in Mars orbit are called "Martian Trojans." Some asteroids have been found inside Earth's orbit (including many Near Earth Objects), while others -- including burnt out or dormant comets, such as pertur bed Edgeworth-Kuiper Belt objects called "Centaurs " -- are located beyond Saturn's orbit. Indeed, many have orbits that cross Earth's path (see orbit diagram of near-Earth Asteroid 4179 Toutatis ), and while small asteroid al fragments hit the Earth every day as meteorites, bigger asteroids are surmised to have landed with impacts that killed off a significant share of life on the planet in times past. While most asteroids may be only the size of pebbles, 16 asteroids have a diameter of 240 km (150 miles) and Ceres, the largest, has a diameter of about about 914 km (568 miles). On August 24, 2006, the International Astronomical Union voted at the end of its 26th General Assembly to establish a new class of substellar objects in the Solar System called "dwarf planets", which may eventually encompass the largest, relatively round asteroids such as Ceres and perhaps eventually Vesta , Pallas , and Hygeia . Mass of the Main Asteroid Belt It has been estimated that the total mass of the Main Asteroid Belt may total less than 1/1000th of the mass of the Earth. Indeed, if all asteroids down to the size of meter- or yard-sized boulders or less were combined together, the resulting object would measure less than 1,300 to 1,500 km (810 to 930 miles) across, which is less than one third to one half the diameter of the Earth's Moon. The Main Asteroid Belt is only a small remnant of the material that once resided in the region between Mars and Jupiter, but once may have contained between two to 10 Earth masses of material . However, T-Tauri-type Solar winds from a very young Sun, gravitational perturbations from Jupiter developing nearby, and dynamic interactions with other large planetesimals and protoplanets during the first 100 million years, and continuing collisional grinding over the following 4.5 billion years after the formation of the planets, interfered with the formation of a substantial, single planet and caused most of the mass to be lost to the rest of the Solar System and interstellar space. Composition Based on the composition of meteorites found on the Earth, most asteroids may be composed of three materials: mostly (92.8 percent) silicates (stone); metals (5.7 percent) iron and nickel; and the rest as a mix of the those materials and carbon-rich substances. Asteroids located closer to Mars and Earth that exhibit the same spectra are composed of rocky minerals ("stone") mixed with iron. In contrast, asteroids located farther away from the Sun on the Jupiter side of the Main Asteroid Belt are generally darker, redder, and more icy, presumably because they were not as well heated by the Sun and so have a composition more like the primordial, circum-Solar dust disk out of which the outer planets accreted about 4.5 billion years ago. Thus, the outer asteroids may more closely resemble the icy planetary bodies of the Edgeworth-Kuiper Belt and Oort Cloud. The larger asteroids The larger asteroids or protoplanets may have accumulated enough internal heat to "differentiate," whereby denser metals settled downwards (and perhaps even form metallic cores) and left lighter rocky ("stoney") residues in their outer layers. On some asteroids, internal heat may also have formed metamorphosized rocks, and volcanoes may even have erupted. Although no asteroid in the Main Belt grew big enough to hold on to an atmosphere, minerals found in some meteorites suggest that liquid water was often present. More *Asteroid Belt - History *List of Large Asteroids *Asteroid Mining Category:Solar Systems